# This code is stored in the file  fden.spl

#================================================#
# f.density.plot()                           		      
# ---------------------------------------------                          
# Input : degrees of freedom; it can be a vector. 
#         (e.g.) f.density.plot(c(1,10,10,50))
#         creates a plot with density curves 	
#         for (1,10) df and (10,50) df        
# Output: density plots of the central F      	    
#         distribution.                     
#
#################################################

f.density.plot <- function(n)
{	
    # draw x,y axis and title
	
    x <- seq(.001,4,,50)
    plot(c(0,4), c(0,1.4), type="n", 
         xlab="x", ylab="f(x;n)",
	 main="Densities for Central F Distributions")

    # the length of df should be even.
    
    legend.txt <- NULL
    d.f <- matrix(n,ncol=2,byrow=T); r <- dim(d.f)[1]
    for(i in 1:r) {
       lty.num <- 3*i-2             # specify the line types.
       f.x <- df(x,d.f[i,1],d.f[i,2])      # calculate density.
       lines(x, f.x, type="l",lty=lty.num)    # draw a line.	
	  legend.txt <- c(legend.txt,
	  paste("(",d.f[i,1],",",d.f[i,2],")",sep=""))
      }

    # The following lines are for inserting 
    # legends on plots using a motif 
    # graphics window.
	
         legend( x = rep(1.9,r) , y = rep(1.1,r) , 
              cex=1.0,
              legend = paste(legend.txt,"df") , 
	       lty = seq(1,by=3,length=r) ,
              bty = "n" )
}


#  This function can be executed with the following
#  commands.  
#
#   par(fin=c(6,7),cex=1.0,mex=1.3,lwd=4)
#   source("fden.spl")
#   f.density.plot(c(1,10,10,50,10,10,10,4))